|Piezoelectric earphones also known
as crystal earphones are the most sensitive earphones. In fact they
are at least 10 times more sensitive than very sensitive magnetic
earphones, dynamic earphones, and electrostatic earphones. Sensitive earphones
have a very high
impedance, which is measured in ohms.
Product Code: CH905ST
A sensitive earphone built around an electromagnet might have
an impedance of about 2,000 ohms, while the impedance of a crystal earphone
is about 20,000 ohms.
A crystal earphone (more properly called a piezoelectric earphone,
pronounced pee-zo) is made of a material that changes its shape when
connected to a source of electricity. Some crystals such as quartz, and
Rochelle's Salt are piezoelectric. Some ceramics (such as those made with
barium titanate) are also piezoelectric. Our piezoelectric earphone is made
of a disk of brass that is coated with barium titanate ceramic. When
electricity is connected to it, the ceramic bends the brass disk, and we can
hear the vibrations this causes in the air.
- Impedance: 20K ohm
- Capacity: 15,000 pf
- Sensitivity: 57 dB at 1KHz
- Frequency Range: 200 ~ 8000 Hz
- Operation Temperature: -20º to 60º
- Cord: 3ft twisted cable.
Test the sensitivity of Crystal Earphone
To demonstrate just how sensitive a crystal earphone is, try this
experiment: with the earphone in your ear, touch the two wires together. You
will hear a sharp click as electrons move from one wire to the other. If the
earphone came with a jack on the end instead of two bare wires, you will
need a piece of metal such as a spoon to connect the two metal parts of the
Use crystal Earphones for Crystal Radio
One detail about such a very sensitive earphone is important in building
a crystal radio. A sensitive earphone does not use very much current to
create the sound. Another way of saying this, is that not much current is
going through the earphone. Our radio needs a certain amount of current to
flow through the diode in order to work.
When substituting a piezoelectric earphone for an earphone made with a
coil of wire, we must provide a way for some current to bypass the earphone.
We do this by putting a resistor or a coil in parallel with the earphone
(parallel means that the resistor or coil is attached to the same two places
that the earphone wires are attached).
The resistor can be anything in the range of 1,000 ohms to 100,000 ohms,
and can be a piece of graphite out of a pencil, or a couple hundred coils of
fine wire around a nail.